Nickel-base alloys have long enjoyed considerable usage as structural materials and have been found to be particularly advantageous when employed in high temperature environments such as those associated with nuclear reactors, jet engines and the like. These nickel alloys exhibit excellent high temperature strength characteristics as well as high resistance to the fluctuating stresses and shock experienced at elevated temperatures. Moreover, many nickel-base alloys are highly resistant to corrosion normally associated with a wide variety of hot gases and corrosive liquids.
In particular, the assignee of this invention produces a high temperature resistant nickel-base alloy called INCONEL alloy 618. (INCONEL is a trademark of the Inco family of companies.) See U.S. Pat. No. 4,153,455 for a complete description of the alloy and its properties. Suffice it to say, this alloy exhibits an excellent combination of high temperature strength and oxidation resistance characteristics. Indeed, it was initially developed to provide structural stability and weldability for High Temperature Gas Cooled Reactor (HTGR) applications.
There is a need in the art to weld oxidation-resistant nickel-base alloys under a variety of industrial conditions. As a filler wire, for example, any alloy selected should be suitable for welding with all major arc-welding processes, e.g., gas tungsten-arc, gas metal-arc, submerged-arc, and be capable of producing sound welded joints under conditions imposing severe restraint and difficult parameters for both thin and heavy section welds. It is also desirable that the alloy be suitable for use as a core wire in a covered electrode.
While sound welds may be made in many instances with matching composition filler material, industry prefers that continuous, high deposition-rate welding processes be employed; said processes requiring however, that the filler metal be readily workable to forms such as wire. Cast filler materials for high production rate welding processes are not economically feasible and it is essential that the filler material be at least hot workable in order to afford the advantages of continuous welding processes. More advantageously, the alloy should also be cold workable, allowing the alloy to be formed to thin cross-sections, e.g., wire, which is the form predominantly used in continuous high deposition-rate welding processes.
Inasmuch as INCONEL alloy 618 (as well as any other alloy) is ultimately joined to something, an attempt was made to develop a welding electrode using the alloy itself as the core wire. Accordingly, standard production INCONEL alloy 618 was utilized as the core wire for the electrode. In this manner, the problems and costs normally associated with producing a new electrode were, hopefully, to be eliminated.
The goal was to develop a welding electrode having a deposit that exhibited the same tensile stress, rupture and oxidation properties as those of the underlying wrought alloy product. Moreover, it was expected that the resulting electrode would exhibit good operability and produce sound weldments.
Initially, it was determined that such an electrode could be produced. However, as developmental work proceeded, it was discovered that an exact duplication of the underlying alloy composition did not produce defect-free weldments. Close examinations of the resulting deposits disclosed evidence of undesirable fissuring. Inasmuch as the alloy was originally developed for high pressure and temperature service applications, there was an urgent need to produce a welding alloy compatible with the superior strength and stability characteristics of INCONEL alloy 618.
Initially, for coated electrodes, modifications were made only in the flux; the alloy composition of the electrode remaining constant. However, it was soon apparent, that the composition of the core wire had to be modified in order to obtain acceptable, defect-free weldments, regardless of whether the wire was coated or not.